1. Field of the Invention
The invention relates to a position sensor for making electric contact in a predetermined relative position of two parts movable relative to each other.
2. Prior Art
Position sensors are known wherein a cam is connected to a rotatable part. A transmission rod guided for longitudinal movement is arranged to be displaced by the cam as a function of the rotary movement. The transmission rod actuates an electric switch, which makes a snap motion between two switch positions. Such known position sensors suffer from disadvantages which are adverse to their use in certain applications.
The cam has a cam lobe arranged to displace the transmission rod. To this end the inertia of the transmission rod has to be overcome. When the sensed rotary movement is very fast, for example with manual actuation, it may happen that the transmission rod will not give way quick enough and the cam lobe will thus strike against the end of the transmission rod, whereby damage either of the cam lobe or of the transmission rod may be caused.
Furthermore, position sensors of this type have an inevitable switching hysteresis due to the snap action of the electric switch. An angular displacement, which is not negligible, exists between the switching-on point and the switching-off point, and, within this range, the switch position of the switch is independent of the position of the cam. The actuating movement of actuating this snap mechanism is transmitted correspondingly to this snap mechanism through mechanical transmission means, i.e., the cam lobe and transmission rod. If the hysteresis of the position sensor were to be reduced by appropriate design of the snap mechanism, this would make sense only, if the precision of the transmission mechanism were increased to the same extent. Only then would an exact relation between the switching point of the snap mechanism and the rotary movement of the cam and of a part connected thereto exist.
These disadvantages of prior art position sensors have adverse affects, for example, when one of the movable parts is the housing, and the other one of the movable parts is the actuator shaft, of a hydrostatic variable-output pump, wherein a signal has to be provided in the zero-stroke position of the variable output pump. In this zero-stroke position, the variable-output pump has a discharge volume of zero. The hydrostatic variable-output pump may be part of a hydrostatic propulsion drive system in a construction machine, for example in a road roller. The hydrostatic variableoutput pump is driven by an internal combustion engine. The fluid under pressure thus delivered by the variable-output pump is supplied to hydraulic motors, by means of which the road roller is driven. The idling position of this propulsion drive is represented by the zero-stroke position of the variable-output pump. In this position, for example, the internal combustion engine may be started. If the variable-output pump is not exactly in its zero-stroke position, when the internal combustion engine is started, there will be delivery of fluid under pressure, whereby the rod roller is driven unintentionally. This may result in accidents. In this case it is desirable to provide an interlocking means which prevents starting of the engine or transmission of the propulsion drive, if the variable-output pump is not exactly in its zero-stroke position.
Another case where a signal indicating the zero-stroke position of the variable-output pump in a road roller or the like is required is in a safety device by which the road roller is braked in an emergency situation. This may be done by shunting the inlet and outlet ports of the variable-output pump and by shutting the ports of the hydraulic motors off. This may, however, also be done by applying a mechanical braking device. In such cases it is necessary to provide a re-activation blocking device, which prevents reactivation of the propulsion drive or releasing of the braking device as long as the control lever for the propulsion, which is connected to the actuator shaft of the hydrostatic variable-output pump, has not yet been returned to its idling position and the variable-output pump is not in its zero-stroke position. In this situation, accidents may be caused by releasing the brake though the variable-output pump has not been returned exactly to its zero-stroke position.
The fault that the starting procedure may be released or the re-activation blocking device may be disabled, with the variable-output pump not exactly in its zero-stroke position, occurs with a position sensor of the prior art type mentioned above, because with the prior art sensor, the setting of the variable-output pump may vary about the zero-stroke position within the hysteresis of the position sensor. It has been found that this fault and the risks caused thereby are not negligible.